In a resist stripping step in semiconductor manufacturing, a sulfuric acid electrolytic method is known, in which cleaning is performed using a peroxosulfuric acid solution as cleaning liquid by electrolyzing a sulfuric acid solution to produce peroxosulfuric acid (peroxodisulfuric acid and peroxomonosulfuric acid; molecular peroxosulfuric acid and ionic peroxosulfuric acid). In the resist stripping step, the resist stripping is promoted more efficiently as the temperature of the cleaning liquid is higher. This is considered to be attributable to that when the cleaning liquid produced by the sulfuric acid electrolytic method reaches a predetermined high temperature, the peroxosulfuric acid in the cleaning liquid self-decomposes to generate a sulfuric radical with extremely high oxidative power, contributing to the cleaning.
However, since the radical has a short life-span, the cleaning liquid is consumed without contributing to the cleaning, when the temperature of the cleaning liquid is raised in an early stage, due to premature self-decomposition of the peroxosulfuric acid contained in the cleaning liquid. When the cleaning liquid is heated slowly over a long time (e.g., about several minutes), a problem in which the concentration of peroxosulfuric acid has been reduced at the moment the cleaning liquid reaches the high temperature arises since the self-decomposition of the peroxosulfuric acid and decomposition of the sulfuric radial associated therewith progress in the middle of heating.
The cleaning of an electronic material substrate or the like is performed by a single-wafer type as well as a batch type. In the single-wafer type, for example, a cleaning object is fixed to a rotary table, and cleaned by means of spraying of a chemical, flushing with the chemical, or the like while rotating the table. In a single-wafer cleaning apparatus, the cleanliness of the electronic material substrate such as a wafer can be kept high, compared with batch type cleaning. Further, an unnecessary resist can be efficiently stripped from an electronic material such as a silicon wafer with relatively little usage of the chemical. However, the chemical to be used in the single-wafer cleaning apparatus is required to have characteristics conditioned more stringently than those of an electrolytic sulfuric acid solution to be used in a batch type cleaning apparatus. In stripping of a resist which is ion-implanted at a high concentration of 1×1015 atoms/cm2 or more, particularly, a cleaning liquid having a further high peroxosulfuric acid concentration and a further high liquid temperature is needed.
In the above-mentioned viewpoint, the present inventors proposed a cleaning system provided with a rapid heating heater (refer to Patent Literature 1) in consideration of the need to perform the temperature rise of cleaning liquid in a given short time just before cleaning. In this cleaning system, a storage tank storing an electrolyte is connected to the rapid heating heater through a supply line, and the outlet side of the rapid heating heater is further joined to a supply line to a cleaning apparatus by a flange or the like. A sulfuric acid solution is sent from the storage tank to the rapid heating heater by a pump, heated by the rapid heating heater, and supplied to the cleaning apparatus as it is at the pressure at the time of solution sending.